Cobie Smulders delivers a career-high performance as a botanist trying to protect some unusual findings in the Room 104 Season 3 finale.

HBO anthology series Room 104 concludes Season 3 with an ethereal and poignant story about a scientist making an otherworldly discovery. When Dr. Marian Wallace (Cobie Smulders) heads to the titular Room 104 for a rest after a long expedition, some stowaway spores lodge into the carpet of the room and take root. Over the next few days an entire ecosystem blossoms, and Dr. Marian sets out on a mission to catalog and conserve. Of course, the government has other plans.

Before the episode aired, we chatted with executive producer Sydney Fleischman about the creation of the episode, entitled “The Specimen Collector”, and learned some fun information about how that wild set was created, how Smulders became involved, and what secrets lie ahead in Room 104 Season 4.

Casting is a key component to Room 104. Often, the series enjoys selecting performers who are well-known for a certain persona, and then turning that persona on its head. (See: June Squibb in “Crossroads”, Rainn Wilson in “Mr. Mulvahill”, Bryan Tyree Henry in “Arnold”, etc.) The casting of Smulders in this episode is no different. Best known for her roles as the bold Robin Scherbatsky in How I Met Your Mother and the heroic SHIELD agent Maria Hill in the Marvel cinematic universe, Smulders shows off another side of her persona here. (Please note that there are no images of Smulders throughout this piece due to her commitment to another series at air time. But we promise she’s actually in the episode!)

Fleischman told us that Smulders became involved in the project due to a professional relationship with writer/director Mel Eslyn. According to Fleischman, the two women had previously worked together on a film called The Intervention, and “just totally hit it off.” Once Cobie was on set, it was clear that she was game for anything. Per Fleischman, “Cobie is one of those people who comes in and asks, ‘what do you need me to do? I’ll do it.’ And she’s especially great in this episode because she’s acting opposite plants as well as special effects that didn’t exist in the room.”

For the majority of the episode’s run time, Smulders is the lone actor on screen. She makes a few brief calls on a cell phone, but mostly viewers witness her marveling at the lush landscape blossoming before her. The room itself is her scene partner. Simply by interacting with the intricate layers of life that appear in the room, she telegraphs a quiet and earnest curiosity that begins to turn desperate as her time runs out.

Room 104 often puts the motel space through some rigorous transformations, but this one may be the most impressive transformation yet. Fleischman says that there were a “lot of logistics” involved in creating that jungle-like ecosystem. The show brought in a greens team to populate the set with “beautiful, lush plants, and then they placed them all around the room to make it feel like they grew there.” Since Room 104 rarely works with a greens team, Fleischman states that the experience of watching the team was inspiring, especially in an episode focused on the future of the planet, stating, “…it was such a cool thing to be able to see how these people come in and take such great care of the plants, especially for this episode which is about stewardship and taking care of the earth.”

If you’ve watched the episode, you know that the creation of the magical jungle was only partially built on practical effects. It was a team effort, with the art department and the VFX team also contributing work to make the ecosystem seem as lifelike as possible. Fleischman also told us that the art department created some custom flowers, and the VFX team “created the insects and the fish in the toilet.” The aim was to “create something that felt really full and lush and complex.”

The team certain succeeded, as Fleischman also told us that the three-and-a-half day jungle shoot became increasingly complicated as the greenery grew in size throughout the timeline. “It was definitely claustrophobic on set,” she said. “[They used] a lot of handheld to basically get the camera in as intimately as they possibly could. Normally the room is actually a decent size and you can move around a fair amount, but with all of the vegetation, there was a restricted mobility.”

Even though the stories of Room 104 are often tinged with horror, this installment feels safe and almost dreamlike. In another episode, one could almost imagine the plants turning on Dr. Marian and doing something horrific, but here there’s just an air of exploration and safety. As Dr. Marian explores, it’s clear that there’s a symbiotic relationship going on here. She wants to protect the trees, and they want to reveal their secrets to her. When asked about how the show successfully switches tone from episode to episode, Fleischman gave some insight. “People are sort of on edge when they turn the show on, and they think that there’s going to be some creepy element or horror, so when there isn’t, that’s really fun for us because it automatically creates this tension. But in an episode like this, you can allow yourself to be comfortable in it […] you’re on your toes, asking what’s going to happen, and what is this? Is she safe? And then once you start to see that she is safe and she is comfortable, then you’re just along for the ride with her.”

However, once the government becomes aware of the mysterious jungle, the tone shifts to one of tension and dread. Aasif Mandvi enters as Dr. Eugene Hill. He’s been sent as a mediator and he seems to be on the same page as Dr. Marian. The themes of climate change and conservation are pervasive throughout this episode. The jungle doesn’t hurt or trick Marian, but the government does.

When asked about the timely message, Fleischman said, “I think it’s something that has been topical for a long time, so it wasn’t a concern about whether or not this was still going to be relevant when it aired or even in five years or ten years. It’s sort of a constant, our urge to conserve things and to explore things and really try to understand something new and how things work and how to protect it. I guess we did see it as topical, but also evergreen.”

While the episode itself will certainly be enjoyed by audiences for years to come, Room 104 Season 4 awaits us. The entirety of the season has already been filmed, and Fleischman teases what’s to come. She says to “expect the unexpected” and promises that Season 4 will continue to “test boundaries and play with limitations.” And, in a tantalizing tease, she let us know that the series is “doing things you haven’t seen on the show before. It’s not what you’re expecting.”

Room 104 Seasons 1 – 3 are currently available for streaming on HBO GO and HBO NOW.